Human dihydropteridine reductase: characterisation of a cDNA clone and its use in analysis of patients with dihydropteridine reductase deficiency

Dihydropyridine Reductase Deficiency: Acute Encephalopathy Related to Folinic Acid Treatment Interruption in a Girl

We reported the case of acute encephalopathy related to colonic acid treatment interruption in a 12-year-old female child presenting to our unit with episodes of vomiting, headache, irritability, acute confusional state, seizures, and left lower limb hypotonia. Brain magnetic resonance imaging (MRI) showed signs of vasogenic and cytotoxic edema at the cerebellar level bilaterally, and lesions at the temporo-occipito-parietal right level, temporomandibular left, and right thalamic with swelling of the convolutions and reduced differentiation between white and gray matter. The patient had suspended the folinic acid treatment at least 6 months before the present admission. The relation between the clinical signs presented by the girl and folic acid deficiency was confirmed by the result of laboratory assessment and by the answer to the notable clinical improvement with the renewal of folinic acid treatment. Dihydropteridine reductase (DHPR) deficiency is a rare autosomal recessive genetic disorder caused by the quinoid dihydropteridine reductase (QDPR) gene mutations. DHPR deficiency impairs the synthesis of the tetrahydrobiopterin (BH4), an essential cofactor for the hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. When not precociously treated, the disorder may present whit severe neurologic impairment including developmental delay/intellective disability (DD/ID), microcephaly, seizures, movement disorders, cerebral palsy, and other neurological impairments. The clinical and neuroradiologic anomalies observed in our case were unusual, with signs previously unreported in patients with folic acid deficiency. The present case shows that the clinical presentation and MRI anomalies of the cerebral folic acid deficiency may be various and unusual compared with those reported in the literature, and it confirms the usefulness of the continuation of folinic acid treatment during the course of the disorder in patients with DHPR deficiency.

Genetics of Phenylketonuria: Then and Now

More than 950 phenylalanine hydroxylase (PAH) gene variants have been identified in people with phenylketonuria (PKU). These vary in their consequences for the residual level of PAH activity, from having little or no effect to abolishing PAH activity completely. Advances in genotyping technology and the availability of locus-specific and genotype databases have greatly expanded our understanding of the correlations between individual gene variant, residual PAH activity, tetrahydrobiopterin (BH4 ) responsiveness, and the clinical PKU phenotype. Most patients (∼76%) have compound heterozygous PAH gene variants and one mutated allele may markedly influence the activity of the second mutated allele, which in turn may influence either positively or negatively the activity of the biologically active heterotetrameric form of the PAH. While it is possible to predict the level of BH4 responsiveness (∼71%) and PKU severity (∼78%) from the nature of the underlying gene variants, these relationships remain complex and incompletely understood. A greater understanding of these relationships may increase the potential for individualized management of PKU in future. Inherited deficiencies in BH4 metabolism account for about 1%-2% of all hyperphenylalaninemias and are clinically more severe than PKU. Almost 90% of all patients are deficient in 6-pyruvoyl-tetrahydropterin synthase and dihydropteridine reductase.

Dihydropteridine reductase deficiency and treatment with tetrahydrobiopterin: a case report

Dihydropteridine reductase (DHPR) deficiency is a genetic disorder of tetrahydrobiopterin (BH4) regeneration and may present with hyperphenylalaninemia, microcephaly, hypotonia, mental retardation, and convulsions. BH4 is an essential cofactor for the hydroxylation of aromatic amino acids and a deficiency of BH4 results in decreased synthesis of dopamine and serotonin. We present a 27-month-old female patient with DHPR deficiency who was treated with L-dopa/carbidopa (2 mg/kg, four times per day), 5-hydroxytryptophan (2 mg/kg, four times per day), folinic acid (10 mg/day), and BH4 supplementation (20 mg/kg, twice a day). Although remarkable clinical improvement with normal plasma phenylalanine (Phe) levels and increased phenylalanine tolerance was noted 1 month after the treatment, CSF neurotransmitter metabolites did not improve. BH4 supplementation was increased to 40 mg/kg/day and the CSF study was repeated 1 month later. There was no significant change of CSF neurotransmitters, BH4 or BH2 levels but plasma Phe level was within normal range. Surprisingly, she had developmental improvement noted at 1-month and 3-month visits following an augmented neurotransmitter and BH4 treatment. She was able to pull herself to the standing position and sit down on her own. She was also noted to be more alert and responsive following treatment. Her expressive language did not improve, although her receptive language was markedly improved. The above treatment improved patient's clinical findings, normalized blood Phe levels, and increased Phe tolerance in the diet, but neither 20 nor 40 mg/kg/day BH4 supplementation corrected neurotransmitter or BH4 levels or increased BH2 level in CSF. Further studies are needed to find the optimal management plan for patients with DHPR deficiency.

Serum prolactin as a tool for the follow-up of treated DHPR-deficient patients

Deficiency of dihydropteridine reductase causes a variant form of phenylketonuria associated with a devastating neurological disease characterized by mental retardation, hypokinesis and other features relating to basal ganglia disorder. Hyperphenylalaninaemias with tetrahydrobiopterin deficiency make up about 1-3% of all hyperphenylalaninaemias. We describe three patients from Calabria, a southern region of Italy, who have a dihydropteridine reductase deficiency, caused by the same mutation (p.L14P) also found in the nearby region of Sicily. We report the evolution of clinical and biochemical data during the treatment of these patients where we used prolactin serum determination to adapt the specific therapy. This report suggests that serum prolactin levels can be a good biomarker for optimal dosage of hydroxylated precursors in long-term treatment monitoring.

Dihydropteridine reductase deficiency in man: from biology to treatment

In 1975, dihydropteridine reductase (DHPR) deficiency was first recognized as a cause of tetrahydrobiopterin (BH(4)) deficiency, leading to hyperphenylalaninemia (HPA) and impaired biogenic amine deficiency. So far, more than 150 patients scattered worldwide have been reported and major progresses have been made in the understanding of physiopathology, screening, diagnosis, treatment, and molecular genetics of this inherited disease. Present knowledge on different aspects of DHPR deficiency, largely derived from authors' personal experience, is traced in this article.

Physarum polycephalum expresses a dihydropteridine reductase with selectivity for pterin substrates with a 6-(1', 2'-dihydroxypropyl) substitution

Physarum polycephalum is one of few non-animal organisms capable of synthesizing tetrahydrobiopterin from GTP. Here we demonstrate developmentally regulated expression of quinoid dihydropteridine reductase (EC 1.6.99.7), an enzyme required for recycling 6,7-[8H]-dihydrobiopterin. Physarum also expresses phenylalanine-4-hydroxylase activity, an enzyme that depends on dihydropteridine reductase. The 24.4 kDa Physarum dihydropteridine reductase shares 43% amino acid identity with the human protein. A number of residues important for function of the mammalian enzyme are also conserved in the Physarum sequence. In comparison to sheep liver dihydropteridine reductase, purified recombinant Physarum dihydropteridine reductase prefers pterin substrates with a 6-(1', 2'-dihydroxypropyl) group. Our results demonstrate that Physarum synthesizes, utilizes and metabolizes tetrahydrobiopterin in a way hitherto thought to be restricted to the animal kingdom.

Tetrahydrobiopterin-deficient hyperphenylalaninemia in the Chinese

BACKGROUND

Hyperphenylalaninemia (HPA) may be caused by either a deficiency in phenylalanine-4-hydroxylase or in tetrahydrobiopterin (BH4), the essential cofactor required for the hydroxylation of aromatic amino acids. The most common forms of BH4 deficiency are 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency (MIM 261640) and dihydropteridine reductase (DHPR) deficiency (MIM 261630), which require a different treatment from classical HPA.

RESULTS

Approximately 86% of BH4-deficient HPA in the Chinese population was found to be caused by PTPS deficiency. Eleven missense (73C-->G, 120T-->G, 155A-->G, 166G-->A, 200C-->T, 209T-->A, 226C-->T, 259C-->T, 286G-->A, 317C-->T, 430G-->C), one splicing (IVS3+1G-->A) and two deletion mutations (116-119delTGTT, 169-171delGTG) were identified in 37 unrelated PTPS-deficient Chinese families. Among these, 155A-->G, 259C-->T and 286G-->A mutation accounted for about 80% of the mutant alleles. The 155A-->G and 286G-->A mutations were found to be the common mutation in southern and northern Chinese, respectively. Only two Chinese DHPR-deficient families were detected among about 300 Chinese hyperphenylalaninemia cases. A single base transition 508G-->A on the DHPR cDNA was identified in two consanguineous DHPR-deficient siblings. A reduced level of DHPR mRNA expression was found in the other DHPR-deficient patient, which suggested that the mutation might lie in the regulatory region of the DHPR gene.

CONCLUSIONS

The BH4-deficient HPA was estimated to make up around 30% of the Chinese population in Taiwan suffering from HPA, which is much higher than in Caucasian populations (1.5-2% of HPA).

Homomeric and heteromeric interactions between wild-type and mutant phenylalanine hydroxylase subunits: evaluation of two-hybrid approaches for functional analysis of mutations causing hyperphenylalaninemia

Phenylketonuria (PKU) is caused by mutations in the phenylalanine hydroxylase gene (PAH), while mutations in genes encoding the two enzymes (dihydropteridine reductase, DHPR, and pterin-4-alpha-carbinolamine dehydratase, PCD) required for recycling of its cofactor, tetrahydrobiopterin (BH(4)), cause other rarer disease forms of hyperphenylalaninemia. We have applied a yeast two-hybrid method, in which protein--protein interactions are measured by four reporter gene constructs, to the analysis of six PKU-associated PAH missense mutations (F39L, K42I, L48S, I65T, A104D, and R157N). By studying homomeric interactions between mutant PAH subunits, we show that this system is capable of detecting quite subtle aberrations in PAH oligomerization caused by missense mutations and that the observed results generally correlate with the severity of the mutation as determined by other expression systems. The mutant PAH subunits are also shown in this system to be able to interact with wild-type PAH subunits, pointing to an explanation for apparent dominant negative effects previously observed in obligate heterozygotes for PKU mutations. Based on our findings, the applications and limitations of two-hybrid approaches in understanding mechanisms by which PAH missense mutations exert their pathogenic effects are discussed. We have also used this technique to demonstrate homomeric interactions between wild-type DHPR subunits and between wild-type PCD subunits. These data provide a basis for functional studies on HPA-associated mutations affecting these enzymes.

Molecular analysis of 16 Turkish families with DHPR deficiency using denaturing gradient gel electrophoresis (DGGE)

Dihydropteridine reductase (DHPR) catalyses the conversion of quinonoid dihydrobiopterin (qBH2) to tetrahydrobiopterin (BH4), which serves as the obligatory cofactor for the aromatic amino acid hydroxylases. DHPR deficiency, caused by mutations in the QDPR gene, results in hyperphenylalaninemia and deficiency of various neurotransmitters in the central nervous system, with severe neurological symptoms as a consequence. We have studied, at the clinical and molecular levels, 17 patients belonging to 16 Turkish families with DHPR deficiency. The patients were detected at neonatal screening for hyperphenylalaninemia or upon the development of neurological symptoms. To identify the disease causing molecular defects, we developed a sensitive screening method that rapidly scans the entire open reading frame and all splice sites of the QDPR gene. This method combines PCR amplification and "GC-clamping" of each of the seven exonic regions of QDPR, resolution of mutations by denaturing gradient gel electrophoresis (DGGE), and identification of mutations by direct sequence analysis. A total of ten different mutations were identified, of which three are known (G23D, Y150C, R221X) and the remaining are novel (G17R, G18D, W35fs, Q66R, W90X, S97fs and G149R). Six of these mutations are missense variants, two are nonsense mutations, and two are frameshift mutations. All patients had homoallelic genotypes, which allowed the establishment of genotype-phenotype associations. Our findings suggest that DGGE is a fast and efficient method for detection of mutations in the QDPR gene, which may be useful for confirmatory DNA-based diagnosis, genetic counselling and prenatal diagnosis in DHPR deficiency.

Dihydropteridine reductase deficiency: physical structure of the QDPR gene, identification of two new mutations and genotype-phenotype correlations

Dihydropteridine reductase (DHPR) is an enzyme involved in recycling of tetrahydrobiopterin (BH4), the cofactor of the aromatic amino acid hydroxylases. Its deficiency is characterized by hyperphenylalaninemia due to the secondary defect of phenylalanine hydroxylase and depletion of the neurotransmitters dopamine and serotonin, whose syntheses are controlled by tryptophan and tyrosine hydroxylases. The DHPR cDNA has been cloned and mapped on 4p15.3. In the present study we report the genomic structure of the DHPR gene (QDPR). This gene includes seven exons within a range of 84-564 bp; the corresponding introns are flanked by canonic splice junctions. We also present a panel of PCR primers complementary to intronic sequences that greatly facilitates amplification of the gene and provides a genomic DNA approach for mutation detection. We have used this approach to study six patients with DHPR deficiency. Four known mutations (G23D, H158Y, IVS5G+ 1A, R221X) and two new mutations (Y150C and G218ins9bp) were found. The Y150C mutation was found in compound heterozygosity with G23D, a mutation always associated with a severe phenotype in homozygous patients. This patient has an intermediate phenotype (good response to monotherapy with BH4). The mutant enzyme for Y150C was expressed in an E. coli system. Comparison of its kinetic parameters with those of the G23D mutant enzyme showed that it is not as effective as the wild-type enzyme, but is more active than the G23D mutant. This patient's intermediate phenotype is thus due to the mild DHPR mutation Y150C. Correlations between genotypes and phenotypes were also found for the other mutations.

A mutation causing DHPR deficiency results in a frameshift and a secondary splicing defect

In our analysis of mutations causing DHPR deficiency we identified a patient in whom there was an aberrant transcription pattern detected by PCR of DHPR cDNA. However, unlike the pattern observed as a result of most splicing mutations, there is some full length transcript. The mutation was located and is a single nucleotide deletion at position 570/571 of the DHPR cDNA sequence and results in a frameshift and premature termination after the addition of six amino acids. The mutation is present in a homozygous state in the patient and in a heterozygous state in both parents. The exon which is deleted at high frequency in the patient is the putative exon 4, which is remote from the mutation, and confirms our observation that exon 4 skipping is a relatively common event.

Molecular basis of dihydropteridine reductase deficiency

The spectrum of mutations causing dihydropteridine reductase is reviewed. A total of 12 point mutations have been described that map in the DHPR cDNA, resulting in amino acid substitutions, insertions and premature terminations. A further two mutations are described which result in aberrant splicing of DHPR transcripts. The application of the mutation identification to diagnostics and clinical treatment is discussed.

Sulfamethoxazole-trimethoprim double-blind, placebo-controlled, crossover trial in Machado-Joseph disease: sulfamethoxazole-trimethoprim increases cerebrospinal fluid level of biopterin

We performed a double-blind, placebo-controlled, crossover trial of sulfamethoxazole-trimethoprim (S-T) in 8 patients with Machado-Joseph disease (MJD), and measured the blood and cerebrospinal fluid levels of biopterins, biogenic amines or metabolites, and folate. The clinical results were as follows; mild improvements of hyperreflexia of knee jerks and of rigospasticity of the legs during S-T treatment period. In addition, S-T significantly reduced the times of 8 motor activities on the timed tests. The biochemical results showed that basal levels of all biopterins and homovanillic acid in the cerebrospinal fluid (CSF) were reduced to less than half the levels of those of controls with other neurological diseases. After S-T treatment, total and oxidized form of biopterins in the CSF increased significantly. Therefore, S-T may be effective to neurologic deficits through its mechanism of increasing the level of brain biopterins.

Structural and mechanistic implications of incorporating naturally occurring aberrant mutations of human dihydropteridine reductase into a rat model

Phenylketonuria (PKU) is a debilitating hereditary disorder related to an individual's inability to convert phenylalanine to its usual tyrosine product. The genetic errors occur in three regions: in the cooperative enzymes phenylalanine hydroxylase (PAH) and dihydropteridine reductase (DHPR), and in the biosynthetic pathway from GTP to the hydroxylation cofactor, tetrahydrobiopterin (BH4). Many instances of naturally occurring defects in DHPR metabolism have been identified, and in most cases the error has been equated with an altered enzyme gene sequence. Using computer graphics, this report analyses the altered structural characteristics of eight of the enzymes encoded by mutant gene sequence and provides logical explanations for their diminished enzyme activities. In one instance, that of a threonine insertion, a mutant construct of the rat analog has been expressed in Escherichia coli and the DHPR isolated and characterised, confirming the marked changes this insert can create.

Hyperphenylalaninemia due to defects in tetrahydrobiopterin metabolism: molecular characterization of mutations in 6-pyruvoyl-tetrahydropterin synthase

A variant type of hyperphenylalaninemia is caused by a deficiency of tetrahydrobiopterin (BH4), the obligatory cofactor for phenylalanine hydroxylase. The most frequent form of this cofactor deficiency is due to lack of 6-pyruvoyl-tetrahydropterin synthase (PTPS) activity, the second enzyme in the biosynthetic pathway for BH4. The human liver cDNA for PTPS was previously isolated, and the recombinant protein was found to be active when expressed in Escherichia coli. We now have investigated two patients for their molecular nature of this autosomal recessive disorder. Both patients were diagnosed as PTPS deficient, one with the central and one with the peripheral form, on the basis of an elevated serum phenylalanine concentration concomitant with lowered levels of urinary biopterin and PTPS activity in erythrocytes. Molecular analysis was performed on the patients' cultured primary skin fibroblasts. PTPS activities were found in vitro to be reduced to background activity. Direct cDNA sequence analysis using reverse transcriptase-PCR technology showed for the patient with the central from a homozygous G-to-A transition at codon 25, causing the replacement of an arginine by glutamine (R25Q). Expression of this mutant allele in E. coli revealed 14% activity when compared with the wild-type enzyme. The patient with the peripheral form exhibited compound heterozygosity, having on one allele a C-to-T transition resulting in the substitution of arginine 16 for cysteine (R16C) in the enzyme and having on the second allele a 14-bp deletion (delta 14bp), leading to a frameshift at lysine 120 and a premature stop codon (K120-->Stop). Heterologous expression of the enzyme with the single-amino-acid exchange R16C revealed only 7% enzyme activity, whereas expression of the deletion allele delta 14bp exhibited no detectable activity. All three mutations, R25Q, R16C, and K120-->Stop, affect evolutionarily conserved residues in PTPS, result in reduced enzymatic activity when reconstituted in E. coli, and are thus believed to be the molecular cause for the BH4 deficiency. This is the first report describing mutations in PTPS that lead to BH4 deficiency.

Identification and in vitro expression of mutations causing dihydropteridine reductase deficiency

Six mutations resulting in the recessive inherited disorder dihydropteridine reductase deficiency are reported, five of which are previously unknown. Two are nonsense mutations, resulting in premature termination of the protein, with the remaining four being missense mutations. The mutations found lie in the middle to 3' end of the dihydropteridine reductase reading frame, with the exception of one mutation which lies at codon 23, which is the only mutation found in more than one patient. The mutation pattern can be described as heterogeneous. The wild type and several of the mutant DHPR cDNA's were expressed in E. coli and the proteins purified and examined by a variety of techniques, including calculation of kinetic constants. One mutation (Gly23-->Asp) results in completely inactive protein, while a second (Trp108-->Gly) has substantial activity but does not completely dimerize. Both this mutant and a third, His158-->Tyr, are extremely susceptible to in vitro protease digestion, indicating that their three-dimensional structure has been altered. The protein studies underline the heterogeneous nature of DHPR mutations, in that the effects of different amino acid substitutions on the DHPR enzyme are varied.

Two new mutations in the dihydropteridine reductase gene in patients with tetrahydrobiopterin deficiency

Two new mutations have been identified within the dihydropteridine reductase (DHPR) gene in two patients with DHPR deficiency. The total coding sequence of the cDNA has been screened by chemical cleavage of mismatch in both patients and selected portions of the cDNA have been sequenced. The first mutation identified causes a glycine to aspartic acid substitution at codon 23 and seems particularly frequent in Mediterranean patients. Its occurrence within a glycine string common to the amino-terminal region in NADH dependent enzymes suggests a possible causal mechanism for the defect. The second change involves a tryptophan to glycine substitution at codon 108 and is carried by both alleles in the second patient. It occurs in a motif which shows similarities with a region of dihydrofolate reductase (DHFR) and is highly conserved within different animal species.

Identity of 4a-carbinolamine dehydratase, a component of the phenylalanine hydroxylation system, and DCoH, a transregulator of homeodomain proteins

The principal pathway for the metabolism of phenylalanine in mammals is via conversion to tyrosine in a tetrahydrobiopterin-dependent hydroxylation reaction occurring predominantly in the liver. Recently, the proposal that certain hyperphenylalaninemic children may have a deficiency of carbinolamine dehydratase, a component of the phenylalanine hydroxylation system, has widened the interest in this area of metabolism. Upon cloning and sequencing the dehydratase, we discovered that this protein is identical to DCoH, the cofactor which regulates the dimerization of hepatic nuclear factor 1 alpha, a homeodomain transcription factor. The identity of the nuclear and cytoplasmic proteins is demonstrated by size, immunoblotting, stimulation of phenylalanine hydroxylase, and dehydratase activity. The evolution of the dual functions of regulation of phenylalanine hydroxylation activity and transcription activation in a single polypeptide is unprecedented.

Dried blood spot on filter paper as a source of mRNA

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PCR primers for human chromosomes: reagents for the rapid analysis of somatic cell hybrids

Rapid analysis of somatic cell hybrids can be facilitated by using the polymerase chain reaction (PCR) to assay for genes assigned to specific human chromosomes. We describe PCR primer pairs for genes on the short and long arms of the 22 autosomes and the X chromosome. Some of the primers were designed from the 3' untranslated region of cDNA sequences, whereas others were derived from genomic sequence. Each primer set was tested for its specificity and mapped to a chromosome by screening a somatic cell hybrid panel. Two of the primer pairs (APOC2 and G6PD) detect CA dinucleotide repeat polymorphisms.

DNA haplotype analyses of patients with hyperphenylalaninemia

Linkage analysis of phenylketonurics has shown a strong association between the DNA haplotype at the phenylalanine hydroxylase (PAH) locus and phenylketonuria (PKU). Similarly, a genetic linkage between less severe forms of hyperphenylalaninemia (HPA) and the PAH locus has been suggested. In the present study we analyzed this linkage in more detail. Haplotypes at the PAH locus were determined for 19 individuals with moderately elevated plasma phenylalanine and normal urinary neopterin/biopterin ratios. Fourteen of these individuals had plasma phenylalanine levels of 4-10 mg/dl (mild HPA), and the other five had plasma phenylalanine levels of 10-19 mg/dl (atypical PKU). Thirteen of the 15 HPA families consisted of an affected child and at least one other sibling. Elevated plasma phenylalanine was seen to genetically segregate with specific PAH alleles in each family. Summation of the LOD scores for both categories of moderate plasma phenylalanine elevation gave a maximum value of 3.556 at theta = 0. At theta = 0 this gives a probability of linkage between the PAH locus and the locus for moderate phenylalanine elevations that is approximately 3,600:1. None of the alleles segregating with either mild HPA or atypical PKU were of haplotype 2 or 3, and 13/20 were of types 1 or 4. This is in agreement with the most deleterious mutations being on haplotypes 2 and 3 and with the less severe mutations being on haplotypes 1 and 4. chi 2 Analyses indicated no statistically significant correlation between HPA and a particular haplotype or restriction-enzyme site.

Insertion of an extra codon for threonine is a cause of dihydropteridine reductase deficiency

The mutation in a patient with dihydropteridine reductase deficiency has been located and characterized. Polymerase chain reaction (PCR) was used to amplify the coding sequence of human dihydropteridine reductase from the messenger RNA of skin fibroblasts. Chemical cleavage of mismatches indicated a mismatched thymine and cytosine at approximately 117 and 147 bases, respectively, from the end of the probe. Cloning and sequencing of the mutant PCR products revealed the insertion of the triplet ACT (threonine), after alanine 122 (base 390). Amplification of a small region around this mutation by using genomic DNA as the PCR target indicates that the mutation is completely within an exon. Unequal crossing-over at the second base in the preceding alanine codon and duplication of the bases CTA may be the mechanism of mutagenesis. The cleavage site 147 bases from the end of the probe corresponded to the conversion of guanine to adenine at base 420 (CTG to CTA) and does not alter the code for leucine. This change, which was also seen in another dihydropteridine reductase-deficient child and in a control subject probably represents a common neutral polymorphism.

Restriction fragment length polymorphisms among Japanese detected with a dihydropteridine reductase cDNA gene probe

Using a human dihydropteridine reductase (DHPR) cDNA, the frequency of restriction fragment length polymorphisms (RFLPs) with restriction endonucleases AvaII, MspI, NcoI and HinfI was estimated in unrelated, unaffected Japanese. The allele frequencies are different from those found in Caucasians, especially with MspI and HinfI. However, approximately 60% of Japanese are heterozygous for at least one of the RFLPs and analysis of one family with DHPR deficiency was shown to be informative. RFLP linkage analysis will be useful in the Japanese population as previously reported for Caucasians.

Anti-idiotypic antibodies elicited by pterin recognize active site epitopes in dihydrofolate reductases and dihydropteridine reductase

Monoclonal antibodies (mAbs) against antipterin immunoglobulin and dihydropteridine reductase (DHPR) and also polyclonal antibodies against human dihydrofolate reductase (DHFR) were obtained. The anti-idiotypic mAbs and anti-DHPR mAbs bind specifically to human DHFR, Escherichia coli DHFR, soybean seedling DHFR, and human DHPR in solid-phase immunoassays. Further, the mAbs bind to the native but not to the denatured forms of DHFRs. The monoclonal antibodies also inhibit the enzymatic activity of human DHFR but not that of human DHPR. Competitive solid-phase immunoassays show stoichiometric inhibition by methotrexate and partial inhibition by NADPH of mAb binding to human DHFR. Cyanogen bromide fragments derived from human DHFR (residues 15-52 and 53-111), containing several active site residues, bind partially to some of the monoclonal antibodies. Accordingly, polyclonal antibodies to peptide 53-111 of human DHFR cross-react to some extent with human DHPR. Data from competitive immunoassays in which the binding of the various mAbs was tested singly and in combination with other mAbs suggest that these antibodies bind to a common region on human DHFR. The results also indicate that the mAbs display some heterogeneity with respect to specific epitopes. These data suggest that despite the absence of significant amino acid sequence homologies among the various DHFRs and DHPR, they have a fundamentally similar topography at the site of binding of the pterin moiety that is recognized by the anti-idiotypic mAbs generated by pterin. In the relatively simple structure of the pterin ring system there are different substituent groups at positions C4 and C6 in methotrexate, 7,8-dihydrofolate, and 7,8-dihydrobiopterin, suggesting that these antibodies are specific for regions on various proteins that interact with the remainder of the pterin moiety. These mAbs and similar mAbs specified by substituent groups on pterin may thus be used as specific probes or inhibitors of various folate-dependent enzymes and transport proteins. They should also provide insights into some of the general features of antibody recognition of protein antigens.

High-level expression of human dihydropteridine reductase (EC 1.6.99.7), without N-terminal amino acid protection, in Escherichia coli

The cDNA coding for human dihydropteridine reductase [Dahl, Hutchinson, McAdam, Wake, Morgan & Cotton (1987) Nucleic Acids Res. 15, 1921-1936] was inserted downstream of tandem bacteriophage lambda PR and PL promoters in Escherichia coli vector pCE30. Since pCE30 also expresses the lambda c1857ts gene, transcription may be controlled by variation of temperature. The recombinant plasmid in an E. coli K12 strain grown at 30 degrees C, then at 45 degrees C, directed the synthesis of dihydropteridine reductase to very high levels. The protein was soluble, at least as active as the authentic human enzyme, and lacked the N-terminal amino acid protection.

Application of DNA-DNA hybridization of dual labeled probes to the detection of trisomy 21, monosomy 21, and sex determination

Chromosomal aneuploidy is usually identified by cytogenetic methods. However, for some purposes it would be desirable to have an easier method of recognizing specific trisomies or monosomies. We have devised such an assay. It involves the simultaneous hybridization of two chromosome-specific DNA probes labeled with different isotopes (32P and 35S) to human chromosomal DNA immobilized on a nitrocellulose filter. The hybridization signals are then counted, and the dpm ratios between the two isotopes are determined. In this paper we show the application of this technique in sex determination, in the analysis of two human cell lines reported to have chromosome 21 monosomy, and, especially, in testing for Down syndrome.

NcoI and HinfI RFLPs detected with a dihydropteridine reductase cDNA probe

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The use of restriction fragment length polymorphisms in prenatal diagnosis of dihydropteridine reductase deficiency

Using a human dihydropteridine reductase (hDHPR) cDNA probe we have detected two AvaII and one MspI restriction fragment length polymorphisms (RFLPs). We show that these RFLPs are in disequilibrium and calculate that approximately 60% of Caucasians are heterozygous for at least one RFLP. We demonstrate the usefulness of these RFLPs in prenatal diagnosis of DHPR deficiency in one family. This disorder can also be predicted by enzyme assays and we therefore discuss the relative merits of the two methods of prenatal diagnosis.

Localization of the human dihydropteridine reductase gene to band p15.3 of chromosome 4 by in situ hybridization

We report the localization of the gene for dihydropteridine reductase (DHPR) to the human chromosome region 4p15.3 by in situ hybridization using a cDNA probe to the enzyme. The distal end of the short arm of chromosome 4 is of considerable interest because the gene responsible for Huntington's disease is located in this region. Although this part of the chromosome is being extensively studied, DHPR is the first well-characterised gene to be assigned to the region. Restriction enzyme fragment length polymorphisms have been detected with a number of restriction endonucleases, including AvaII and MspI. These features may make the DHPR cDNA clone a useful probe not only for prenatal diagnosis of DHPR deficiency but also for linkage studies of Huntington's disease.